In the production of polyvinyl chloride by suspension polymerization, the monomer or monomer mixture is finely distributed in the aqueous phase by mechanical agitation, stabilized with the aid of protective colloids, and polymerized in the presence of radical initiators.
Usable protective colloids or suspension stabilizers are water-soluble, natural, polymeric substances, such as gelatin, water-soluble cellulose derivatives, such as alkyl celluloses, hydroxyalkyl celluloses, and the like, water-soluble synthetic products, such as partially saponified polyvinyl acetates, copolymers of maleic anhydride or vinylpyrrolidone with vinyl acetate, and also various combinations of the aforementioned materials.
In suspension polyvinyl chloride resins produced according to conventional processes, there is a correlation between the porosity and the bulk density of the resin, in that an increase in porosity to achieve satisfactory demonomerization results in a reduction in bulk density. However, a decrease in bulk density means a diminished output of extruded polyvinyl chloride resin. In processing PVC without plasticizer to obtain a hard product, it is desirable to maintain a high bulk density and a maximally high porosity since thereby devolatilizability of the polyvinyl chloride resin is facilitated, the mixing time for introduction of the processing aids and thermostabilizers prior to extrusion is shortened, and the number of gels or fisheyes in the extrudate is reduced.
In order to achieve both high porosity and bulk density, surfactants have been added as porosity-improving agents, such as, for example, sorbitan monolaurate (cf DOS No. 2,208,796, which disclosure is incorporated by reference herein). However, polyvinyl chloride resins produced according to this method exhibit an inadequate bulk density, a reduced extruder output during processing without plasticizer, and a high level of gels in the extrudate.
It has also been known to use, in suspension polymerization of vinyl chloride, polyvinyl alcohols or, respectively, cellulose ethers as primary suspension agents, and high-molecular weight compounds containing nitrogen and oxygen atoms as the cosuspension stabilizers, of DOS 2,234,038, British Pat. No. 991,134, DOS No. 2,653,087, which disclosures are incorporated by reference herein. As can be seen from the comparative tests set forth in the table, these polyvinyl chloride resins show, however, a comparatively low output during extrusion without plasticizers, or a very high level of gels in the extrudate.